This invention is directed to improved lens molds for the production of contact lenses, intraocular lenses, and other ophthalmic products. In particular, the invention involves protective coatings for allowing for the use of mold materials having improved dimensional stability and/or increased light transparency. The invention is also directed to a method of making the improved lens molds and their use in the manufacture of contact lenses.
The molds used in the manufacture of soft (hydrogel) contact lenses have been made from a variety of rigid thermoplastic resins. For example, U.S. Pat. No. 5,540,410 to Lust et al and U.S. Pat. No. 5,674,557 to Widman et al. disclose mold halves made from polystyrene, polyvinyl chloride, polyethylene, polypropylene, copolymers of polystyrene with acrylonitrile and/or butadiene, acrylates such as polymethyl methacrylate, polyacrylontrile, polycarbonate, polyamides such as nylons, polyesters, polyolefins such as polyethylene, polypropylene and copolymers thereof, polyacetal resins, polyacrylethers, polyarylether sulfones, and various fluorinated materials such as fluorinated ethylene propylene copolymers and ethylene fluoroethylene copolymers. Polystyrene is preferred by Widman et al because it does not crystallize and has low shrinkage. An earlier patent, U.S. Pat. No. 4,661,573 to Ratkowski et al, discloses, for the processing of fluorosilicone copolymers into extended wear lenses, molds formed of polypropylene, polyethylene, nylon, Teflon(copyright), glass, or aluminum having its mold surfaces coated with Teflon(copyright) polymer.
The manufacturers of soft contact lenses have discovered that if the molds used to make the lenses are sufficiently inexpensive, it is more economical to discard the molds after production of the lenses from the molds than it is to clean the molds to be reused. Polypropylene is a good example of an inexpensive resin that has been used to make molds that can be discarded at minimal cost. Another advantage of polypropylene is that unlike many resins, polypropylene can resist interaction with the monomers used to make the contact lenses. The ability to resist chemical interaction prevents the lens and the mold from adhering to each other and simplifies their separation following lens production.
Despite these benefits, however, polypropylene lens molds also suffer from several known disadvantages. One disadvantage is polypropylene""s relatively low dimensional stability. As mentioned by Widman et al, polypropylene partly crystallizes during cooling from the melt and is, therefore, subject to shrinkage, causing difficulties in controlling dimensional changes after injection molding. To improve dimensional stability, manufacturers can make polypropylene lens molds thicker. However, while thicker polypropylene molds can have greater stability, they also require additional cooling time. The additional time needed to cool the thicker molds decreases the number of molds that can be made per machine per unit of time. Furthermore, thicker and therefore larger polypropylene molds can limit the number of molds per machine, thereby reducing product throughput. Finally, polypropylene""s relatively poor dimensional stability limits manufacturing yield, because the molds may need to be stored before use, for periods of up to several weeks in some cases, and many polypropylene molds fail to maintain dimensional stability over time to a degree that eventually renders them unfit for lens production.
In addition to having relatively poor dimensional stability, polypropylene has other disadvantages. Polypropylene is a translucent resin that reduces the transmission of light. Typically, polypropylene allows only about ten percent of light to pass through it. Poor light transmission reduces the speed of polymerization. Furthermore, the absorption of oxygen by the molds, commonly experienced with polypropylene molds, can influence lens quality. When the absorbed oxygen diffuses out, during lens molding, polymerization can be affected, and lens surface quality can suffer as a result.
Several alternative resins offer greater dimensional stability and light transmittance than polypropylene. For instance, polycarbonate and polystyrene are more amorphous resins and, therefore, have greater dimensional stability than polypropylene. Moreover, these and other xe2x80x9cclearxe2x80x9d resins generally transmit at least 50% and often more than 70% of light.
Although polycarbonate and polystyrene resins offer greater dimensional stability and light transmittance, they are vulnerable to chemical interaction with the monomers used in many soft contact lenses (for example, N-vinylpyrrolidone and N,N-dimethylacrylamide, used in many conventional contact lenses). Chemical interaction between the lens monomers and the lens molds can cause the lens and the mold to adhere to each other and, in a worst case scenario, the lens and the mold can become permanently joined. Moreover, in addition to being susceptible to chemical interaction, many clear resins are more expensive than polypropylene and are, therefore, too costly to discard.
Molds for making soft contact lenses have been treated to affect their surface properties. For example, U.S. Pat. No. 4,159,292 discloses the use of silicone wax, steric acid, and mineral oil as additives for plastic mold compositions to improve the release of the contact lens from the plastic molds. U.S. Pat. No. 5,690,865 discloses an internal mold release agent such as waxes, soaps, and oils, including a polyethylene wax having a molecular weight of 5,000 to 200,000 or a silicone polymer having a molecular weight of 2,000 to 100,000. U.S. Pat. No. 5,639,510 to Kindt-Larsen discloses a surface-applied surfactant in the form of a uniform layer or very thin film or coating to assist in the release from each other of mold components of a multi-part mold employed in the molding of hydrophilic contact lenses. Polymeric surfactants that can be used include polyoxyethylene sorbitan mono-oleates which are applied to a non-optical surface of the mold, but do not cover the optical surface of the mold.
U.S. Pat. No. 5,674,557 to Widman et al discloses hydrophilic contact-lens molds that are transiently modified with a removable surfactant to provide a low water dynamic contact angle. This was found to reduce lens hole defects in lens manufacture. Widman et al discloses various polysorbates, ethoxylated amines, or quaternary ammonium compounds that can be applied to the mold surface by swathing, spraying, or dipping.
Mueller et al, in European Patent Application EP 0 362 137 A1, discloses the coating of molds with a co-reactive hydrophilic polymer like polyvinylalcohol, ethoxylated PVA, or hydroxyethyl cellulose, in order to provide a permanent hydrophilic coating on the lens. The mold coating copolymerizes with the lens material in the mold. Similarly, Merill, in U.S. Pat. No. 3,916,033, discloses coating the surface of a mold with polyvinylpyrrolidone to form a coating that is later to come into contact with a previously crosslinked silicone lens. Merill teaches spreading a coating solution over the mold while held in a chuck, thereby achieving a fairly uniform coating of several thousandths of an inch, after which the wet film is allowed to dry to form a hard glassy polymer layer of about 1 to 5 thousandths of an inch. Finally, monomeric N-vinyl pyrrolidone is dissolved in the coating ready for contact with the silicone lens. As one other example, U.S. Pat. No. 5,779,943 to Enns et al. discloses coating a mold with a hydrophobic latent-hydrophilic material, after which a lens material is molded therein. During curing, the mold coating is apparently transferred to the lens surface. The lens is then treated to convert the coating to a hydrophilic form.
It is an object of our system to provide the manufacturer of contact lens and other ophthalmic articles placed on or in the eye with an improved way to mold them, by providing molds with greater light transparency or dimensional stability which can be stored for an extended period of time while, concurrently, maintaining the chemical resistance of the molds to a variety of monomers used in making the ophthalmic articles. This combination of mold properties can be economically achieved by use of the present invention, and may even be achieved in molds that are discarded after a single use.
Our system uses clear-resin lens molds with protective coatings for the production of contact lenses, intraocular lenses, delivery devices, and other such ophthalmic articles. Clear resins are not only more amorphous and, therefore, more dimensionally stable than polypropylene, but are also capable of transmitting a greater percentage of actinic light. Various clear resins are suitable for contact lens molding purposes, including polyvinyl chloride (PVC), polyester, polysulfone, polyacrylate/polymethacrylate, polycarbonate, and polystyrene.
Polycarbonate, polystyrene, or polyacrylate materials are particularly preferred. These resins offer great dimensional stability and light transmittance; but unlike many other clear resins, they are also relatively inexpensive. Despite the advantages associated with clear-resin lens molds, it is commonly known that clear resins have a tendency to interact with monomers used in contact lens production. Until now, manufacturers have only been able to develop a limited number of clear-resin lens molds that can effectively resist chemical monomer interaction. Unfortunately, existing clear-resin lens molds may be too expensive, especially to discard after single-use, or may only offer resistance to limited kinds of monomer materials used in the manufacture of ophthalmic articles.
The present invention is directed to clear-resin molds and methods of producing clear-resin molds, used for making ophthalmic articles, having a permanent (non-transient), dense, uniform, and continuous protective inorganic coating on the surface of the mold, including at least the optical surfaces thereof, to prevent adverse monomer chemical interactions. Although several coating techniques are capable of applying such a coating, including evaporation, sputtering, spraying, and photo-chemical vapor deposition, the preferred embodiment for applying a protective coating to the mold employs plasma-enhanced chemical vapor deposition (PECVD) or other plasma glow discharge techniques, which make such coatings generally uniform, continuous, dense, and pinhole free. The coating, during molding of the ophthalmic article, is essentially inert or non-reactive with the lens monomers or lens surface that is formed in the mold assembly. On the contrary, the purpose of the coating is to prevent chemical interaction of the mold with the polymerizable monomers used in making the opthalmic article.
A variety of inorganic coating materials that can be used to prevent monomer chemical interaction with the underlying mold surfaces. Suitable coatings include silicon and metal oxides, carbides, and nitrides. Of these, SiOx, SiON, Si3N4, TiO2, Ta2O5, and Al2O3 have been found to be particularly effective. These inorganc coatings, which do not contain carbon as a principal element, can have minor amounts of carbon or H and may be formed from carbon-containing compounds or precursors.
Although the above-listed coating materials are preferred because of their ability to transmit actinic radiation and to protect against monomer interaction, there are other materials that, despite their diminished capacity to transmit light, can be used to prevent monomer interaction. Furthermore, materials for contact lenses, for instance, can also be cured by sources of energy other than light (for example, by heat or microwave energy wherein transmission of light is unnecessary for curing). In such cases, metal nitrides such as TiN or AlN or metal carbides such as TiC or SiC, which do not transmit light, can be used to protect against monomer interaction with the mold material. Moreover, even when employing light to cure a lens material, it may not be essential that light be able to transmit through both female and male mold parts since light is generally directed from only one side of the lens mold. Therefore, in cases where light for curing a lens or other article only needs to be directed from one side, mold materials with limited light transmittance can be applied only to the mold cavity surfaces through which light transmission is not essential. For example, by coating one mold part with a light-transmitting coating material and the other with a coating material having poor light transmittance, manufacturers can produce a clear-resin lens mold that is well suited for contact lens production.
Although a single coating can be used to protect the cavity surfaces of mold parts, multiple coatings formed from different materials can also be used. Even if more costly to apply multiple coatings to the mold cavity surfaces, the presence of multiple layers may provide enhanced protection against monomer interaction and may facilitate release of a molded article from the mold. For example, anywhere from one to five layers or more of an inorganic coating can be economically applied to a mold part. Also, besides preventing monomer interaction with clear resins used to make mold parts, additional advantages of significance can also be obtained by use of mold coatings according to the present invention. For example, mold coatings according to the present invention can also be used to improve mold separation, prevent adhesion, and achieve preferential release.
By eliminating the risk of chemical interaction, manufacturers can mold contact lenses or other ophalmic products using clear-resin molds with improved dimensional stability and light transmittance. Moreover, the application of a coating to the mold cavity surfaces can also accomplish preferential release. For example, our system simplifies and economizes lens production by reducing mold cycle time, increasing product throughput, improving lens quality, and increasing the speed of polymerization.
These and other objects of the invention can be better understood by reference to the Sole Figure in combination with the following detailed description of the invention.